How to use one object to cut a hole into another one
Use the Knife Project tool.


  Blender Manual
  To use Knife Project, first while in Object Mode, select the “cutting object” then select the “object to be cut”. Now, enter Edit Mode and press Knife Project (found in the Mesh > Knife Project).


When you're done, just delete the "cutting object" and also any unwanted faces from the cut object.